The present invention relates generally to gas-filled overvoltage surge arresters, and more particularly to a gas-filled overvoltage surge arrester, which is essentially comprised of two axially opposed end electrodes, a middle electrode disposed concentrically thereto, and of two hollow cylindrical insulators disposed between the middle electrode and the end electrodes.
Gas-filled overvoltage surge arresters having two end electrodes and a concentric middle electrode, so-called three-electrode arresters, are used in different power classes. One of several characteristic features of each power class is the nominal a.c. discharge current that the arrester must be able to conduct, for example, for one second at 50 Hz or for 11 periods (cycles) at 60 Hz. Discharge currents of this type are on the order of 2.5 to 10 to 20 A (1 sec/50 Hz) per arrester gap; for arresters of a higher power class (heavy duty) on the order, for example, of 90 A (11 cycles/60 Hz) for each arrester gap simultaneously; and in the highest power class (maximum duty) more or less on the order of 200 A (11 cycles, 60 Hz) simultaneously for each arrester gap.
In known three-electrode arresters for the highest power class, the two end electrodes have a pin-type formation and widen in the shape of a crown at the mutually opposing ends. The top area of these electrodes and a section of their shafts are surrounded by a concentrically arranged, tubular middle electrode, into whose ends is inserted in each case a hollow cylindrical insulator, which is radially connected via one part of its peripheral area in a gas-tight manner to the middle electrode. Mounted on the other ends of the two insulators in each case is a metal cap, which, on the one hand, is likewise connected in a gas-tight manner with one section of the peripheral area of the particular insulator, and which, on the other hand, is soldered or welded in a gas-tight manner to the shaft of the particular end electrode. An axial length of about 45 to 50 mm (e.g., see U.S. Pat. Nos. 3,289,027, 3,885,203, and GB-A 2 181 887) is characteristic in the highest power class for commercially available specific embodiments of these known three-electrode arresters.
It is also known, especially with regard to the requirements of the North American market, to provide the mentioned three-electrode arresters with supplementary devices, which, in the event of a too intense heating of the arrester, short-circuit the arrester (fail-safe) or, in the event the arrester becomes leaky, activate an auxiliary discharge gap (vent-safe). For these purposes, one uses members of low-melting metals or meltable insulating foils (e.g., see U.S. Pat. Nos. 4,062,054, 4,212,047, and 3,254,179).
For the above-mentioned lower power classes, three-electrode arresters are also known, in which both the end electrodes, as well as the middle electrode are made of copper, and in which the end electrodes are provided with a flange-type base, while the middle electrode has the form of a hollow cylindrical ring with a radially extending mounting flange. The two hollow cylindrical insulators of the arresters are soldered, on the one hand, at the front ends, to the base of one end electrode and, on the other hand, to the connecting flange of the middle electrode. Also, in such arresters, it is customary to provide the insulators on their inner surface area with ignition (or conductive) strips, which are alternately electrically interfaced with one end electrode and with the middle electrode, and to coat the two end electrodes and/or the middle electrode with an activating compound (e.g., see U.S. Pat. Nos. 4,433,354, and 4,768,736). In commercially available specific embodiments of these known three-electrode arresters having copper electrodes, the cylindrical section of the end electrodes has a diameter of about 2 to 3 mm and an axial length of about 3 to 4 mm, so that the cylindrical section of the end electrodes has a volume of about 20 mm.sup.3. The wall thickness of the hollow cylindrical ceramic insulators lies at 0.8-1.5 mm. For three-electrode arresters of this type, one has likewise already provided to develop them in the lower power range with reliable "fail-safe" and/or "vent-safe properties". For this purpose, one can affix a two-arm spring clip, whose ends fit on the front side on the end electrodes of the overvoltage surge arrester, a spacer being disposed therebetween, on the mounting flange of the middle electrode. The end of each arm of the spring clip thereby bears a cap, which is provided with a flange-like rim that serves as a contact ring. This flange-like rim and the base of the particular end electrode oppose one another axially and are kept apart by means of a disk-shaped fusible pellet and a cylindrical component having insulating properties. Either a temperature-resistant insulator or a metal-oxide varistor are used as a cylindrical component (e.g., see U.S. patent application, Ser. No. 08/128,422 filed on Sep. 29, 1993 by the assignee of the present invention). In addition, for the radial contacting of the copper end electrodes of a three-electrode arrester, it has already been proposed to provide the rim existing at the base, at the front end, with a contact ring of a weldable material, whose coefficient of thermal expansion amounts to about 120.times.10.sup.-7 /.degree. C., and to weld a connecting wire (or attachment lead) to the outer surface of this contact ring (e.g., see U.S. patent application, Ser. No. 08/290,274 filed on Aug. 15, 1994 by the assignee of the present invention).
The present invention is directed to the problem of developing a gas-filled overvoltage surge arrester with two cylindrical, axially opposing end electrodes made of copper and provided with a flange-like base and having a middle electrode, likewise made of copper, surrounding the two end electrodes over one section of their axial length, in which a hollow cylindrical insulator with a wall thickness of about 1 mm is disposed between the middle electrode and the bases of the end electrodes in each case, which overvoltage surge arrester has the most compact possible construction, i.e., the smallest possible axial and radial dimensions, of the power class "maximum duty" (200 amp. a.c. discharge current each side to ground simultaneously, 11 cycles, 60 Hz), which, without any substantial increase in size, can also fulfill the requirements "fail-safe" and/or "vent-safe".